Nasa has been forced to delay the launch of its first space shuttle mission since the 2003 Columbia disaster after engineers discovered a problem with one of Discovery’s fuel sensors.

The announcement, just over two hours before the shuttle’s scheduled launch, comes on the heels of another embarrassing hitch, when technicians had to repair a broken window cover that damaged the shuttle's delicate thermal tiles yesterday.

Today's problem was with one of the four engine cut-off sensors, which are responsible for making sure the spacecraft’s engines shut down at the proper point during the ascent.

Nasa said that it appeared that the sensor was showing a low fuel level, even though the tank was full. If the engines were to continue running at high speed without fuel, it could lead to a catastrophe.

"For some reason [the sensors] did not behave today and so we’re going to have to scrub this launch attempt," Mike Leinbach, the launch director, said. No new date was given for a rescheduled attempt.

It was not the first time these sensors malfunctioned. During a fuelling test of Discovery’s original tank in April, some of the sensors gave intermittent readings.

The launch would have been a nervous climax to 30 months of reviews, modifications and handwringing at Nasa, where technicians and scientists have struggled to ensure the safety of the shuttle programme after Columbia broke up 200,000ft above Texas on February 1, 2003.

Just two weeks ago, a panel of scientists and former astronauts appointed to monitor safety improvements said that Nasa had not done enough to meet three crucial recommendations made in the aftermath of the disaster.

Columbia disintegrated as it re-entered the Earth's atmosphere after sustaining damage to its thermal tiles on take-off. The broken thermal shield then allowed the shuttle to heat up dramatically and fall to pieces.

And yesterday, in a frightening repetition of that accident, a temporary window cover fell off Discovery as it waited on the launch pad, and damaged some thermal tiles.

But this morning, the two tiles were replaced and fuelling began. Then a team of technicians made a thorough check of the shuttle to remove any ice that might have formed on the external fuel tanks as a result of the super-cooled fuel.

Ice falling from the tanks on lift-off is thought to be as dangerous as the piece of foam that damaged the wing of Columbia and broke its thermal tiles.

Shown on Nasa television, the crew of Discovery seemed relaxed today as they waved to their families and made their way to the launch pad. Michael Griffin, Nasa Administrator, had earlier offered his reassurance that the shuttle was safe.

"I think it is as safe as we know how to make it be, and there are a lot of unknowns remaining in spaceflight," said Dr Griffin, who promised that he would have flown on Discovery if he could.

The families of the seven astronauts killed on Columbia also added their best wishes for the flight and praised Nasa for making safety improvements to the shuttle.

The Columbia families said they were still grieving for the lost astronauts but that they "know the exploration of space must go on."

"We hope we have learnt and will continue to learn from each of these accidents so that we will be as safe as we can be in this high-risk endeavour," they said in a statement. "Godspeed, Discovery."

Discovery's planned mission is the 114th space shuttle flight in 24 years. The shuttle has a modified external fuel tank and nearly 50 other improvements that were made in the wake of the Columbia tragedy.

During the planned 12-day mission, the crew will dock at the international space station and test various ambitious techniques for repairing the shuttle while in orbit. The astronauts will also learn how to use a 50ft pole which is designed to help them inspect the shuttle.

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